Digital data transmission between systems and devices is prevalent across many technologies. Data may be assembled at a specific clock rate at the transmitting device and transmitted to a receiver to receive the data and recover the data at the clock rate in which the digital data stream was assembled and/or transmitted. Some devices capable of transmitting digital data streams to other devices often do so without sending the accompanying clock signal used to assemble/transmit. Instead, the receiver may be configured to generate an appropriate clock signal for data recovery starting from an approximate frequency reference, and then analyzing the received data to decipher the actual clock frequency that the transmitter used. Such a referenceless clock and data recovery (CDR) scheme then alleviates the need to use bandwidth and/or additional signal lines for the data clock signal between transmitter and receiver.
To this end, Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that have been established for multiple digital bit streams transmissions over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). Conventional CDR schemes involve passing the digital data stream through a phase-locked loop (PLL) such that the transitions in the received digital stream (e.g., the transitions from logic 1 to logic 0 and vice versa) are analyzed to decipher and lock in the clock rate in which the data may be recovered. In order for a CDR scheme to work, the digital data stream must transition frequently enough to correct for any drift in the PLL's oscillator. Further, in order to meet industry standards, the digital data stream between the transmitter and receiver must be greater than specific threshold bandwidths. For example, the Optical Carrier 192 SONET Specification for optical communication requires bit rates of greater than 9 GBPS. Thus, in order to meet this bit rate, a referenceless CDR in a receiver should have a jitter tolerance (JTOL) of at least +/−0.375 UI.